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Cationic UV Adhesives Laminate Opaque Films via Dark-Cure

Compared to solventborne adhesives, cationic UV adhesives offer the advantages of low volatile organic content (VOC) and low odor. UV adhesives contain almost no VOCs. Compared to conventional, solventless adhesives, cationic UV adhesives offer the advantages of reduced cure time (one day for cationic UV vs. three to 14 days for conventional solventless), lower-cost processing equipment and low viscosity, which allows the cationic UV adhesives to be applied at room temperature. Cationic adhesives are also free of isocyanates.

Cationic UV coatings and inks are used for decorating difficult substrates, such as plastics and metals, because of their excellent adhesion, which has been attributed to the low shrinkage of the epoxide ring-opening polymerization (see Figure 1).

Cationic UV adhesives typically contain epoxides, polyols and cationic photoinitiator. The cationic photoinitiator releases an acid when exposed to UV light. The acid catalyzes epoxide polymerization, and hydroxyls react with polymerizing epoxide chains (see Figure 1).1 The acid remains active for some time after it is released by the photoinitiator, allowing cure to continue and properties to improve with time. This phenomenon of continuing cure is known as the “dark cure” effect.

Cationic UV Dark Cure for Opaque Films

Cationic dark cure can be put to unique advantage for UV laminating adhesive applications. Cationic UV adhesives can be specially formulated to laminate opaque substrates that cannot be laminated using other UV chemistries. A properly formulated cationic adhesive can be applied to a web and exposed to UV light just before nipping two films together. The cationic dark cure continues after UV exposure, and adhesive properties, such as T-peel strength, improve with time. Figure 2 shows equipment arrangements for UV-curing before and after nipping.

Films such as clear polyester, white films, metallized films and aluminum foil limit or prevent UV-light transmission. Figure 3 shows the UV transmission of some clear films for comparison. Almost all the UV light passed through the clear polyethylene (PE) and oriented polypropylene (OPP) films. The PE and OPP may have contained very small amounts of or no chromophores that absorbed UV light.

On the other hand, the clear polyester (PET) prevented transmission of about half of the UV light (see Figure 3). The PET (poly[ethylene terephthalate]) polymeric backbone contains aromatic terephthalate chromophores that absorb UV light. Thicker PET would absorb more UV light because the light would have to pass through more chromophores. White films, metallized films and aluminum foil would prevent transmission of all of the UV light.

In order to deliver sufficient UV light to the adhesive when using UV to laminate opaque substrates, the UV cure must occur before laminating, as shown in the press schematic in Figure 2a. Figure 2b is a press schematic for UV curing after laminating, which can be used for films that are transparent to UV light.

A properly formulated cationic adhesive intended to be UV-cured before nipping must be tacky just before nipping, and the properties must build after nipping. Certain ingredients can be used in cationic adhesives to achieve this balance. The relative amounts of epoxides and polyols and their chemical compositions affect both cure rate and adhesive properties.2 Cycloaliphatic epoxides polymerize faster than aliphatic epoxides and glycidyl ethers in the presence of acid catalyst.

Cycloaliphatic epoxides are the preferred materials for cationic UV applications because of their fast cure, low viscosity and low odor. However, cycloaliphatic epoxides cure too fast to be used in adhesives intended to be UV-cured before nipping unless cure retardants are included in the formulations. Polyurethane and polyether polyols and some nitrogen-containing polymers retard cationic cure because they contain nucleophilic groups that decrease the activity of the acid catalyst. Such materials can be used in cationic adhesives that are to be UV-cured before nipping.3,4,5

Results

Some polyurethane polyols can contribute tack in addition to retarding cationic cure.2,3
Adhesive A in Table I (page 35) contained a polyurethane diol that was used to improve tack after UV cure. It helped adhere the films (OPP) together and slowed cure enough so that the films could be pressed together in the lab after exposing the adhesive to UV in a sequence analogous to the press schematic shown in Figure 2a. The T-peel strength of adhesive A continued to improve after laminating because of the dark-cure effect
as shown in Figure 4. Within 24 hours, the peel strength of A increased dramatically.2

Since cationic coatings containing epoxidized polybutadiene typically have good adhesion to plastics, the polyurethane diol in A was replaced on a weight basis with epoxidized polybutadiene to give Adhesive B in Table I. Adhesive B cured too fast to be UV-cured before laminating in the lab because the epoxidized polybutadiene did not retard cure. Therefore, B was laminated before UV curing in the lab according to the press schematic depicted in Figure 2b. The T-peel strength of B was excellent using OPP as shown in Figure 5.

Effects of the Epoxy:Polyol Ratio

The effects of the epoxide and polyol concentrations on T-peel strength were determined using UVR 6128, a cycloaliphatic epoxide that contributes excellent adhesive properties and flexibility, as shown in Figure 6, and a polyether triol in adhesives C, D and E in Table II. The relative epoxide and polyol concentrations in C, D and E are reported in Table II as the R-value, which represents the ratio of epoxide equivalents to polyol hydroxyl equivalents.

Figure 7 shows that D, which had an R value of 2, had greater T-peel strength than C and E, which had R values of 1 and 4, respectively, in PE laminates. Adhesives C, D and E were also low-viscosity (see Table II), which was beneficial for applying them at room temperature. The adhesives were odorless and contained almost no VOCs. The data in Figure 7 further demonstrates the cationic dark-cure effect on T-peel strength.

The flexible cycloaliphatic epoxide resin, UVR 6128, in D was replaced with a harder, less flexible cycloaliphatic epoxide, UVR 6110 (see Figure 6), at the same R value (not the same weight percent) while using the same polyol and designated F in Table II. Adhesive F, which contained UVR 6110, had poorer T-peel strength after 24 hours than D, which contained UVR 6128, as shown in Figure 7, when the laminates contained PE and were UV-cured before laminating.

The stiffer UVR 6110 probably decreased the ability of the adhesive to dissipate energy during the T-peel test resulting in earlier failure. Formulation optimization could possibly improve the T-peel strength using UVR 6110 and the polyether triol.

The effects of other ingredients on T-peel strength were also determined using PE film. Adhesive G, containing UVR 6128 and a vinyl resin (see Table II), had good T-peel strength as shown in Figure 7. The vinyl resin also increased tack after UV-curing and before laminating.

Formulations containing two glycidyl ether epoxides, a conventional liquid bisphenol A diglycidyl ether (H) and dipropylene glycol diglycidyl ether (I), both of which retarded cure, gave poor T-peel strength when used with UVR 6128 (see Figure 7) and PE film. Perhaps the formulations cured too slowly because of the high glycidyl ether epoxide resin concentrations. Adhesives H and I had very little tack after UV cure. Further optimization may uncover formulations containing the glycidyl ether epoxide resins that would improve tack and T-peel strength.

In some cases, the T-peel strength decreased with time (see Figure 7). Decreasing T-peel strengths could indicate that the adhesives became too brittle as they dark cured. This points out the importance of both optimizing the formulations and measuring the properties as a function of time when designing cationic adhesives. (Previously, an experimental design was demonstrated to be a useful timesaving formulating tool for maximizing cationic adhesive T-peel strength.2 Variable T-peel strengths in Figure 7 were probably the result of experimental error because the standard deviations of the T-peel measurements were typically large relative to the results.)

Effects of Film on T-Peel

Adhesives A and B had much higher T-peel strength when the laminates contained OPP rather than PE, as shown in Figure 5. Adhesive A was UV-cured before laminating, and B was UV-cured after laminating. On the other hand, C (Figure 7) had excellent T-peel strength when the laminate contained PE, indicating that the adhesive needs to be optimized for the film type to maximize T-peel strength. The T-peel strength of C was not tested using OPP.

UV-Curing After Laminating

Cationic UV adhesives containing polyester polyols were previously shown to have high T-peel strengths when UV-cured after laminating (see Figure 2b). Adhesives containing UVR 6110 and polyester polyol generally cured too fast to be UV-cured before laminating.2 Figure 8 shows T-peel strengths of fast-curing adhesives J, K and L from Table III that were UV-cured after laminating. Adhesive J, which had the lowest R value (R 4), had the highest T-peel strength. Polyols generally improve the flexibility of cationic formulations. The higher T-peel strength at higher polyol concentration was probably another reflection of improved flexibility increasing the energy dissipation efficiency of the adhesives. This further demonstrated that the adhesive chemist must optimize the concentrations of ingredients used in a specific adhesive.

Effect of Web Speed on UV Dose

Finally, it must be mentioned that the UV dose was found to be an important factor affecting performance of UV adhesives.2 Figure 9 shows the effect of conveyor speed on dose using a conveyorized lab UV-cure unit. The conveyor speed is directly comparable to the web speed on a press. A relatively low UV dose of 70 mJ/cm2 was used for these experiments. The converter would be well advised to work closely with the adhesive supplier to be sure enough UV energy will be available at the desired press speed for acceptable adhesive performance. The amount of UV dose required to get acceptable properties may very well be specific to the adhesive formulation. More UV lamps, higher powered lamps or both would likely be necessary at press speeds greater than 250 ft/min in order to deliver sufficient UV dose to the adhesive.

Experimental

The conveyorized UV-cure unit was from Fusion UV Systems, Inc., and it was operated using a conveyor speed of 250 ft/min and one 300 W/in microwave-activated mercury-vapor (“H”) lamp. The UV dose, which was the amount of UV energy available per area, was measured by passing a radiometer through the conveyorized UV-cure unit. The radiometer was an “IL 390B Light Bug” from International Light, Inc. The UV dose under these conditions was 70 mJ/cm2. Millijoules (mJ) per square centimeter are the units used for reporting the amount of UV light available. The UV transmission of films was measured by taping film samples over the radiometer sensor and then passing the radiometer covered with film through the UV unit.

The adhesives were applied to 6-in x 10-in pieces of film in the direction of film orientation (machine direction) using a 2.5 wire-wound bar. The adhesives were UV-cured by passing the samples once through the conveyorized UV-cure unit using the conditions described.

In an attempt to mimic UV-curing the adhesives before nipping on a press, adhesives were applied to films and the samples were passed through the UV unit. Immediately after exiting the UV unit, 6-in x 10-in pieces of film were placed on the adhesives, and the laminate samples were pressed together using a rubber hand-brayer. To simulate UV curing after nipping the films on a press, the laminates were constructed, pressed together with the brayer and then passed through the UV unit.

Samples 1 in x 9 in were cut from the laminates at different times after UV curing, and T-peel strengths were measured. A computerized Instron Model 1122 instrument was used to measure T-peel adhesive strengths following ASTM method D1876-95.6
The samples were pulled at a rate of 10 in/min. Four or five samples were tested for each set of conditions, and average peel strengths were reported.

Conclusions

It was shown that specially formulated cationic UV adhesives can be used to laminate substrates that are not completely transparent to UV light. Dark cure is epoxide polymerization occurring after UV exposure and catalyzed by acid released by the photoinitiator during UV exposure. Ingredients such as polyurethane and polyether polyols, which slightly retard cationic cure, were used to achieve the balance of increased open time and tack. This allowed the adhesives to be UV-cured before laminating and sufficient dark cure, which allowed properties to build after nipping the films together. The T-peel strengths were demonstrated to increase with time in properly formulated adhesives, which was attributed to the well-known cationic dark cure. Cationic UV adhesives were demonstrated to have high T-peel strengths. The cationic UV adhesives exhibited low viscosity and low odor. The type of plastic film had a significant effect on laminate T-peel strength. The importance of formulation optimization and UV dose was stressed.

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